It is known in the plastic molding art to use pressurized fluid in conjunction with the plastic molding of articles. The pressurized fluid is typically nitrogen gas which is introduced into the mold cavity with the molten plastic.
The pressurized fluid serves at least two purposes. First, it allows the article so formed to have hollow interior portions which correspond to weight and material savings. Second, the pressurized fluid within the mold cavity applies outward pressure to force the plastic against the mold surfaces while the article sets. This is desirable toward enhancing surface quality by minimizing sink marks in areas of the article having relatively thicker plastic sections, such as structural ribs.
However, the added equipment and process control parameters required to implement fluid injection contribute significantly to the cost and complexity of this type of injection molding.
Another concern is that when the gas and plastic are injected into the mold from the same nozzle, the gas gets trapped in the nozzle area thereby foaming any molten resin in that area during venting. This foamed plastic causes blemishes (i.e. splay) in the next part when injected into the mold.
Of particular concern are the fluid charging components and circuits needed to charge, inject and vent the pressurized fluid at the controlled times and pressures necessary for quality part production on a repeatable, volume basis. Prior art devices addressing this concern have answered with complex valves and seals which are expensive, difficult to operate, and require maintenance or replacement at frequent intervals. The problem is especially acute with seals whose performance diminishes over their useful life.